1. Field
The present disclosure relates to thermoelectric materials and methods of preparing the same, and more particularly, to thermoelectric materials having a high degree of orientation and methods of preparing the thermoelectric materials.
2. Description of the Related Art
Thermoelectric materials allow creation of a voltage, when there is a different temperature on each side of the material, based on the thermoelectric effect. The thermoelectric effect refers to direct conversion of a temperature difference into an electric voltage and vice versa, caused by migration of charge carriers, specifically, electrons and holes, in a material. The Seebeck effect, which is a conversion of a temperature difference directly to electricity, may be applied to provide a power generation system by using an electromotive force created by a temperature difference between two ends of a material. The Peltier effect, which is a phenomenon wherein heat is evolved at an upper junction and absorbed at a lower junction when a current flows through a circuit, may be applied to provide a cooling system by using a temperature difference between two ends of a material, wherein the temperature difference is formed by supplying current to the material. The Seebeck effect and the Peltier effect are different from Joule heating in that the Seebeck effect and the Peltier effect are thermodynamically reversible.
Currently, thermoelectric materials may be applied to active cooling systems of semiconductor equipment or electronic devices in which thermal management may not be entirely addressed using a passive cooling system. Further, there is an increasing demand for thermoelectric devices in fields wherein thermal management may not be entirely solved by existing refrigerant-based gas compression systems, such as precision temperature control systems applied to DNA analysis. Thermoelectric cooling is an environmentally friendly cooling technology that does not cause vibration or noise and does not use a refrigerant gas, which causes environmental problems. High-efficiency thermoelectric cooling materials may be applied to commercially available cooling systems such as refrigerators and air conditioners. Thermoelectric materials are regarded as a potential reproducible energy source, because if a thermoelectric material is applied to heat dissipating parts of automotive engines, industrial plants, or the like, power may be generated due to a temperature difference between ends of the thermoelectric material. Such thermoelectric power generation systems have already been used in spacecraft travelling to Mars, Saturn, and regions in which solar energy is not available.
The performance of a thermoelectric material may be evaluated by using a dimensionless figure of merit ZT as defined by Equation 1 below.ZT=S2σT/κ  (1)
In Equation 1, S is the Seebeck coefficient of the material, σ is the electrical conductivity of the material, T is the absolute temperature of the material, and κ is the thermal conductivity of the material. As shown in Equation 1, in order to increase the dimensionless figure of merit ZT, the Seebeck coefficient S and the electrical conductivity σ, specifically, a power factor S2σ, should be increased and the thermal conductivity k should be decreased. In particular, in order to increase the electrical conductivity σ of the thermoelectric material, both high carrier concentration and high carrier mobility are desirable. Although the carrier concentration may be increased by selecting constituents of the thermoelectric material, if the carrier concentration is increased, the thermal conductivity k is also increased. Accordingly, because there is a limit to improving the thermoelectric performance by increasing the carrier concentration, it is more desirable to improve thermoelectric performance by increasing the carrier mobility, i.e., by increasing the electrical conductivity σ.
If the number of grain boundaries in the thermoelectric material is reduced, the proportion of carriers scattered at the grain boundaries is also reduced, thereby increasing the carrier mobility. The number of grain boundaries may be reduced by preparing a highly crystalline material (a single crystal would have no internal grain boundaries), or by increasing an average grain size of the thermoelectric material. Examples of well-known single crystal growth methods include a Bridgeman method, a Czochralski method, and a Zone melting method. However, because the aforesaid methods are difficult to control, take a lot of time, and because the mechanical strength of the thermoelectric material decreases as the average grain size increases, it is difficult to use the aforesaid methods to grow a single crystal.
In order to solve the above and other problems, attempts have been made to increase the carrier mobility by sintering a polycrystalline powder at a high temperature and pressure by using hot pressing, or the like, to increase the degree of orientation. However, high-pressure sintering methods, such as hot pressing, fail to provide a thermoelectric material having a sufficiently high degree of orientation as compared to a degree of orientation of a material including single crystals.